Heat-shrinkable, plastic tubing has many uses. Polyolefin heat-shrinkable tubing is used in packaging food. Tetrafluoroethylene polymer heat-shrinkable tubing, because of its chemical resistance and electrical properties, is placed around elastomeric rolls used in printing presses and copying machines and is also used for putting a snugly fitting coating on electrical connections. Generally in these applications, the tubing is placed around the article to be covered and then heat shrunk to form a tight fit around the article.
In the art, heat-shrinkable plastic tubing can be made by radially expanding lengths of plastic tubing to a larger diameter within a rigid cylinder such as a metal or glass pipe, heating the pipe externally and pressurizing the interior of the plastic tubing to cause it to radially expand against the interior walls of the pipe, cooling, and then releasing the pressure. The plastic tubing so prepared retains sufficient orientation to produce a substantial, e.g., 30%, radial shrinkage when it is placed around an article and heated, which causes it to shrink tightly around the article.
The foregoing art procedure has several disadvantages:
Firstly, the rigid pipe permits only one size diameter plastic tubing to be made. If different size plastic tubing is wanted, another size pipe must be fabricated and used to replace the pipe already fixed in the apparatus. In the invention described herein, this disadvantage is obviated by employing a heat-resistant tubular-like or cylindrical, flexible restraint in place of the pipe whose diameter can be rapidly and easily adjusted. Thus, a single apparatus can be used to make a variety of diameters of the plastic tubing.
Secondly, heating the rigid pipe in order to heat the plastic tubing is time consuming. Moreover, initially the tubing is in contact with pipe only at the bottom where it rests against the pipe and this causes non-uniform heating of all the portions of the tubing and the blowing out of hot spots. These disadvantages are overcome by heating the tubing while it is being pressurized by forcing a hot gas into the interior of the tubing.